Impressed current cathodic protection

ABSTRACT

An impressed current cathodic protection arrangement includes an elongate metallic structure to be protected and cathodic protection apparatus which comprises a DC power supply and an anode. One terminal of the power supply is connected to the structure at a connection point and another terminal of the power supply is connected to the anode. The arrangement includes monitoring apparatus for monitoring effectiveness of cathodic protection provided by the cathodic protection apparatus by determining the electrical potential of the structure relative to surroundings at at least one location which is spaced from the connection point.

This invention relates to impressed current cathodic protection arrangements.

Impressed current cathodic protection arrangements are commonly used to protect metallic structures against corrosion. The systems work by applying a negative potential to the metallic structure which is to be protected such that corrosion processes are suppressed. This is achieved by the use of a DC power supply which has one terminal connected to the structure to be protected and another terminal connected to an anode. The system will only be effective in preventing corrosion provided that the structure is held at a large enough negative potential relative to the surroundings. For practical reasons the impressed current will be applied to the protected structure at a finite number of locations. In some circumstances this may be a single location.

It will be clear that as one progresses along the metallic structure away from the connection point, the magnitude of the potential of the protected structure relative to the surroundings will decrease. If a point is reached where the effective potential of this structure relative to the surroundings is less than a required threshold level, the cathodic protection will cease to be effective from that point onwards.

Impressed current cathodic protection systems are used to protect a range of different metallic structures. One particular situation where they are used is in the protection of flow-lines, for example pipeline systems and well insulations in the oil and gas industry. In some such circumstances direct inspection or direct measurement of the conditions at various points along a protected structure can be impractical or impossible.

This situation is likely to be particularly acute with impressed current cathodic protection systems which are to protect elongate metallic structures such as pipelines where the impressed current needs to be effective over a significant distance of metallic structure leading away from the connection point.

This invention is directed at addressing these types of issues.

According to one aspect of the present invention there is provided an impressed current cathodic protection arrangement comprising an elongate metallic structure to be protected, cathodic protection apparatus which comprises a DC power supply and an anode, one terminal of the power supply being connected to the structure at a connection point and another terminal of the power supply being connected to the anode, and monitoring apparatus for monitoring effectiveness of cathodic protection provided by the cathodic protection apparatus by determining the electrical potential of the structure relative to surroundings at at least one location which is spaced from the connection point.

The monitoring apparatus may comprise means for determining the electrical potential of the structure relative to surroundings at a reference point on the structure to be protected which reference point is spaced from said one location. The reference point may be the connection point.

The means for determining the electrical potential of the structure relative to surroundings at the reference point may comprise a second structure which is spaced from the structure to be protected and the anode, and means for measuring the potential difference between the second structure and the structure to be protected to allow determination of the electrical potential of the structure relative to surroundings at the reference point.

The means for determining the electrical potential of the structure relative to surroundings at the reference point may comprise a memory in which is stored a parameter which is representative of the impedance of the elongate structure as seen from the reference point, means for measuring the current supplied to the structure at the reference point and means for calculating the electrical potential of the structure relative to surroundings at the reference point using said parameter and the measured current.

The elongate structure to be protected may comprise tubing. Typically the tubing may be tubing provided in an oil and/or gas installation such as a well and/or pipeline system. In such a case the reference structure may comprise another well or pipeline installation.

Note that here the expression tubing is used to refer to casings, liners and any other such tubular metallic structure found in a well installation as well as production tubing, for which the term “tubing” is sometimes reserved in the oil and gas industries. The elongate structure may comprise two or more tubing structures running at least partly within one another.

The monitoring apparatus may comprise a central station.

The central station may comprise part or all of the means for determining the electrical potential of the structure relative to surroundings at the reference point.

The central station may be arranged to determine the electrical potential of the structure relative to surroundings at at least one location which is spaced from the connection point in dependence on one or more inputs.

The monitoring apparatus may comprise a tool which is arranged to run within the tubing. The tool and central station may be arranged for communication therebetween.

The tool may comprise a spaced pair of contacts for contacting with an internal surface of the tubing at axially spaced locations, and a control unit comprising a sensor for measuring the potential difference between the contacts. The control unit may comprise a transmitter for transmitting, to the central station, data indicating a measured potential difference between the contacts.

Preferably the tool is arranged to transmit said data along the elongate metallic structure. Other transmission mechanisms may be used, for example a cable may be provided along which said data may be transmitted, or non-electrical techniques may be used.

The monitoring apparatus, for example the central station, may be arranged to determine the electrical potential of the structure relative to surroundings at at least one location which is spaced from said connection point in dependence on the potential difference measured between the spaced contacts of the tool as measured by the sensor of the tool.

Preferably the tool is arranged for movement within the tubing. This can allow potential difference measurements to be taken at a plurality of positions along the structure. The monitoring apparatus may be arranged to use the tool to take potential difference measurements continuously or at intervals as it is moved along the tubing. This can allow a curve of potential along the tubing to be plotted.

The monitoring apparatus may comprise a memory in which is stored a model based on parameters specific to the elongate structure and its surroundings which model predicts how the absolute value of a potential applied to the structure at the reference point will decay along the length of the structure away from the reference point.

The parameters on which the model is based may comprise one of, or a combination of: the resistivity of the material of the elongate structure, the relative permeability of the material of the elongate structure, the cross-sectional dimensions of the elongate structure, the resistivity of material surrounding the elongate structure, the relative permeability of material surrounding the elongate structure. These parameters may he recorded as a function of position along the elongate structure. That is to say the value of these parameters need not be constant along the whole length of the elongate structure. The model may take into account the presence of multiple lengths of tubing arranged within one another, some of which extend further than others. Thus, for example, the model may be based on parameters concerning lengths of casing as well as production tubing in a well installation.

The monitoring apparatus may be arranged to use the model based on parameters specific to the elongate structure and its surroundings in determining the electrical potential of the structure relative to surroundings at at least one location which is spaced from said connection point.

The means for determining the electrical potential of the structure relative to surroundings at the reference point may comprise a memory in which is stored a model based on parameters specific to the elongate structure and its surroundings which model predicts how the absolute value of a potential applied to the structure at the reference point will decay along the length of the structure away from the reference point.

The monitoring apparatus may be arranged to determine the electrical potential of the structure relative to surroundings at at least one location which is spaced from said connection point in dependence on:

-   i) the electrical potential of the structure relative to     surroundings at said reference point as determined by the means for     determining the electrical potential of the structure relative to     surroundings at the reference point; and in dependence on at least     one of: -   ii) the potential difference measured between the spaced contacts of     the tool as measured by the sensor of the tool; and -   iii) the model based on parameters specific to the elongate     structure and its surroundings.

Potential difference measurements from the tool and/or made at the reference point can be used to help verify and/or improve the accuracy of the potential difference determined using the model at any point.

The monitoring apparatus may be arranged to indicate when the magnitude of the potential of the structure relative to the surroundings falls below a threshold level at at least one point along the length of the structure. The threshold level may be a minimum acceptable level for cathodic protection purposes.

The monitoring apparatus may be arranged to indicate at what distance from the reference point the potential of the structure relative to the surroundings falls below a threshold level.

The monitoring apparatus may be arranged to indicate the potential of the structure relative to the surroundings as a function of distance away from the reference point.

According to another aspect of the present invention there is provided a well installation comprising an impressed current cathodic protection arrangement as defined above.

In such a case the central station may be provided at the surface of the well and the tool may be provided to run within the tubing of the well. The central station may be arranged to determine the potential of the tubing of the well relative to the surrounding at the reference point and to determine the potential of the tubing of the well at locations away from the connection point in dependence on:

-   i) the potential difference measured between the spaced contacts of     the tool as measured by the sensor of the tool; and -   iii) the model based on parameters specific to the elongate     structure and its surroundings.

According to another aspect of the present invention there is provided a method of monitoring an impressed current cathodic protection arrangement, the arrangement comprising an elongate metallic structure to be protected, and cathodic protection apparatus which comprises a DC power supply and an anode, one terminal of the power supply being connected to the structure at a connection point and another terminal of the power supply being connected to the anode, the method comprising the step of:

-   monitoring effectiveness of the cathodic protection provided by the     cathodic protection apparatus by determining the electrical     potential of the structure relative to surroundings at at least one     location which is spaced from the connection point.

The method may include further specific steps corresponding to the features defined above, with appropriate changes in wording where necessary. These features are not restated here in method form in the interest of brevity.

Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 schematically shows a well installation including an impressed current cathodic protection arrangement;

FIG. 2 shows an example plot of the absolute electrical potential of an elongate conductor protected by an impressed current cathodic protection system against position along that conductor; and

FIG. 3 is a flowchart showing steps in a method of monitoring an impressed current cathodic protection arrangement.

FIG. 1 shows a well installation including an impressed current cathodic protection arrangement. The well installation comprises a wellhead 1 and tubing 2 running away from the wellhead and down into the well. The tubing 2 is a metallic structure and it is this structure which is to be protected by impressed current cathodic protection in the present case.

Note that FIG. 1 shows a simplified schematic view of the well installation, in particular of the metallic tubing running downhole. Only one run of tubing is shown. In reality there will normally be additional casings (i.e. lengths of metallic tubing) running into the well as well as the central string of production tubing. In general terms these lengths of tubing contact against one another at multiple locations and hence in practical terms will have the same potential as each other. Thus their presence does not interfere with the operation of the present techniques and the tubing 2 shown in FIG. 1 can be considered to be representative of the castings as well as the production tubing of the well.

The impressed current cathodic protection arrangement shown in FIG. 1 comprises cathodic protection apparatus 3 which is used to provide an impressed current onto the metallic tubing 2 of the well so as to protect the tubing against corrosion. The cathodic protection apparatus 3 comprises a DC power supply 31 and an anode 32. A first terminal of the DC power supply 31 is connected to a connection point 33 on the tubing 2 of the well. A second terminal of the DC power supply 31 is connected to the anode 32. Thus the DC power supply is arranged for applying cathodic protection currents. In this regard the cathodic protection apparatus 3 of the present arrangement is conventional and the tubing 2 is protected against corrosion by virtue of the negative potential applied to it using the cathodic protection apparatus 3.

However the present impressed current cathodic protection arrangement further comprises monitoring apparatus 4 for monitoring the effectiveness of the cathodic protection provided by the cathodic protection apparatus 3. In the present embodiment the monitoring apparatus is distributed. That is to say, it has various components provided in different locations. In the present embodiment the monitoring apparatus 4 includes a central station 5 provided at the surface of the well and in the same region as the power supply 31 of the cathodic protection apparatus 3 and a downhole tool 6 which is provided within the tubing 2 of the well. Also provided as part of the monitoring apparatus 4 is a remote reference earthing structure 1′ which, in this embodiment, is in the form of a wellhead of another well. This other well is remote from the well having tubing 2 which is to be protected by the cathodic protection apparatus 3.

The central unit 5 comprises a monitoring unit 51 including a memory 52 and a processor 53. The central unit 5 also comprises a meter 54 for detecting the current applied by the power supply 31 of the cathodic protection apparatus 3 to the connection point 33. Furthermore the central unit 5 comprises a meter 55 for detecting the potential difference between the connection point 33 and the reference wellhead 1′. The outputs of both of these meters 54,55 are fed into the monitoring unit 51.

The memory 52 of the monitoring unit 51 has stored in it a parameter which is representative of the impedance of the elongate conductor to be protected (i.e. the tubing 2) as seen from a reference point, in this case the connection point 33.

Using the supplied current value detected by the meter 54 and this stored parameter relating to the impedance of the tubing 2 it is possible for the monitoring unit 51 to determine the potential at the connection point 33. Thus this is one way in which the present monitoring apparatus 4 is able to determine the electrical potential of the connection point 33 (reference point) relative to the surroundings.

However the potential of the reference point/connection point 33 in the present embodiment may also be directly determined from the meter reading of the meter 55 arranged for measuring the potential difference between the connection point 33 and the remote wellhead 1′. This is because it is reasonable to assume that the potential of the remote wellhead 1′ is the same as the potential of the surroundings in the region of the connection point 33. This common potential is akin to “earth”.

Thus the monitoring unit 51 may determine the potential of the connection point 33 relative to the surroundings using either of these methods or both of these methods.

However for the present purposes what is of primary interest is the potential of the metallic structure 2 at locations away from this connection point 33.

The memory 52 of the monitoring unit 51 also contains a model which is representative of the way in which an absolute potential applied to the metallic structure at a reference point (i.e. in this case, the connection point 33) will decay as one progresses along the metallic structure (in this case, down into the well).

This model is based on parameters relating to the resistivity of the material of the elongate structure (i.e. of the tubing 2—both casings and production tubing), the relative permeability of the material of the elongate structure, the cross-sectional dimensions of the elongate structure, the resistivity of the material surrounding the elongate structure and the relative permeability of the material surrounding the elongate structure. Furthermore these parameters may be recorded as a function of position along the elongate structure. Using these parameters the model predicts how potential will decay along the metallic structure and a typical plot of how the potential will decay along the metallic structure is shown in FIG. 2.

Using this model and the results of the determination of the potential at the connection point 33, the monitoring unit 51 is able to predict the potential of the metallic structure 2 relative to its surroundings at different positions along the length of the metallic structure 2.

In particular it is possible to therefore determine if the magnitude of the potential of the metallic structure 2 falls below an acceptable threshold level for providing cathodic protection at any point along the metallic structure 2.

The monitoring apparatus 4 can be arranged to indicate if this occurs and/or to output a plot which is indicative of the potential of the tubing 2 along its length.

Further it is then also possible to alter the current delivered by the DC power supply of the cathodic protection apparatus 3 so as to raise the magnitude of the cathodic protection current being supplied at the connection point 33 and hence hopefully raise the magnitude of the potential of the tubing 2 to acceptable levels along the entirety of its length (or the entirety of the length of the tubing 2 which is to be protected). The monitoring apparatus 4 can be arranged to perform such a function.

As mentioned above, in the present embodiment the monitoring apparatus 4 also comprises a downhole tool 6. In the present embodiment the downhole tool 6 is arranged for movement within the tubing 2. The tool 6 comprises axially spaced contacts 61. A first of the contacts 61 is provided at one end of the tool 6 and a second of the contact 61 is provided at the other end of the tool 6. These contacts are arranged for contacting with the internal surface of the tubing 2. The tool 6 comprises control unit including a meter 62 for sensing the potential difference between this pair of contacts 61 and hence the potential difference between two axially spaced points on the tubing 2. Furthermore the control unit of the tool 6 comprises transmission means 63 for transmitting potential difference readings back to the central unit 5.

Different forms of transmission between the tool 6 and central unit 5 may be used. In some instances a direct cable connection may be provided, in other instances acoustic or other wireless transmission techniques may be used. A particularly preferred wireless transmission technique is one which makes use of the metallic structure itself (in this case tubing 2) as a signal channel. The applicants supply commercially a tool under the name CATS, which can provide such a transmission mechanism in an embodiment of the present kind. This tool works on the basis of supplying a very high current low frequency electrical dipole signal to the metallic structure 2 via the contacts 61 such that the signal propagates away from the tool 6 and can be picked up from or in the region of the wellhead 1.

Because the spacing between the space contacts 61 is known, the potential difference measurement which can be taken by the tool 6 can be used to determine the slope of the potential curve of the tubing 2 in the region of the tool. Therefore provided that the position of the tool 6 within the tubing is known, knowledge of this slope of the potential curve can be used to help calibrate the model and the plot of potential versus depth in the well (as shown in FIG. 2) to improve the accuracy of potential determination at points away from the connection point 33.

As an alternative, where the tool 6 is arranged for movement within the tubing 2 it is possible to take potential difference readings at multiple points within the well, for example, beginning from the region of the connection point 33. In such a case it is possible to plot a curve representing the potential of the tubing 2 relative to the surroundings using the determined value for the absolute potential at the connection point 33 as a start point and the potential difference readings from the tool 6 to plot the curve from there onwards. In such a case there is no absolute requirement to use the model contained in the memory 52, which describes the decay of potential along the tubing.

FIG. 3 is a flowchart illustrating methods of monitoring an impressed current cathodic protection arrangement of the type shown in FIG. 1. The steps of these methods broadly correspond to the techniques described above.

Step 1 of the method is to determine the potential of the tubing at a reference point (in the embodiment described above, the connection point 33) relative to the surroundings.

Step 2 is to determine a potential decay curve along the tubing 2 as one moves away from the reference point.

Step 3 is to determine whether the magnitude of potential falls below an adequate level for effective cathodic protection at some point along the length of the protecting metallic structure (the tubing 2 in the above embodiment).

Step 4 is an optional step of determining the point at which the magnitude of the potential of the tubing falls below the acceptable level.

Step 5, which again is an optional step, is to adjust the current applied by the power supply of the cathodic protection apparatus in order to ensure that the potential of the tubing does not fall below an acceptable level for cathodic protection purposes.

In carrying out step 1, when operating the embodiment of FIG. 1, it is possible to use the meter 54 for measuring the current delivered by the power supply 31 of the cathodic protection apparatus and the stored characteristic impedance of the tubing from the memory 52 to determine the potential at the reference point. Similarly it is possible to use the potential difference measured by the meter 55 between the connection point 33 and the reference wellhead 1′. Furthermore it is possible to use a combination of both of these results to decide a value for the potential at the reference point 33. Thus, for example, an average of the two values could be used.

Note that it is not possible to directly measure the potential difference at the connection point 33, relative to the wellhead 1 or any other structure in the region of the anode 32, since when the cathodic protection apparatus is in operation (which is when it is desired to measure the potential at the connection point 33) the potential in the region surrounding the anode 32 will be heavily influenced by the cathodic protection system itself.

When conducting step 2 using the apparatus of the embodiment shown in FIG. 1, it is possible to use the model of the decay of the potential along the metallic structure (tubing 2) stored in the memory 52 and similarly it is possible to use the results of the potential difference measurements made by the tool 6. Again, furthermore it is possible to use a combination of these two techniques to determine the potential decay curve.

As mentioned above, whilst the above description has been written in terms of a cathodic protection system provided in a well installation, the present apparatus and method is similarly applicable to any system with an elongate conductor and particularly so for an installation including a tubular elongate metallic structure within which a tool can pass. Thus another particular example are pipeline systems, particularly those used in the oil and gas industry. Note, of course, however that the present systems, where appropriate, can be used without the inclusion of a tool 6 which is located within, or runs within the structure to be protected. 

1. An impressed current cathodic protection arrangement comprising: an elongate metallic structure to be protected, cathodic protection apparatus which comprises a DC power supply and an anode, one terminal of the power supply being connected to the structure at a connection point and another terminal of the power supply being connected to the anode, and monitoring apparatus for monitoring effectiveness of cathodic protection provided by the cathodic protection apparatus by determining the electrical potential of the structure relative to surroundings at at least one location which is spaced from the connection point.
 2. An impressed current cathodic protection arrangement according to claim 1 in which, the monitoring apparatus comprises equipment for determining the electrical potential of the structure relative to surroundings at a reference point on the structure to be protected which reference point is spaced from said one location.
 3. An impressed current cathodic protection arrangement according to claim 2 in which the reference point is the connection point.
 4. An impressed current cathodic protection arrangement according to claim 2 in which the equipment for determining the electrical potential of the structure relative to surroundings at the reference point comprises a second structure which is spaced from the structure to be protected and the anode, and equipment for measuring the potential difference between the second structure and the structure to be protected to allow determination of the electrical potential of the structure relative to surroundings at the reference point.
 5. An impressed current cathodic protection arrangement according to claim 2 in which the equipment for determining the electrical potential of the structure relative to surroundings at the reference point comprises a memory in which is stored a parameter which is representative of the impedance of the elongate structure as seen from the reference point, equipment for measuring the current supplied to the structure at the reference point and equipment for calculating the electrical potential of the structure relative to surroundings at the reference point using said parameter and the measured current.
 6. An impressed current cathodic protection arrangement according to claim 1 in which the monitoring apparatus comprises a central station.
 7. An impressed current cathodic protection arrangement according to claim 6 in which the central station comprises part or all of the equipment for determining the electrical potential of the structure relative to surroundings at the reference point.
 8. An impressed current cathodic protection arrangement according to claim 6 in which the central station is arranged to determine the electrical potential of the structure relative to surroundings at at least one location which is spaced from the connection point in dependence on one or more inputs.
 9. An impressed current cathodic protection arrangement according to claim 1 where the elongate structure to be protected comprises tubing and the monitoring apparatus comprises a tool which is arranged to be located within the tubing.
 10. An impressed current cathodic protection arrangement according to claim 9 in which the tool comprises a spaced pair of contacts for contacting with an internal surface of the tubing at axially spaced locations, and a control unit comprising a sensor for measuring the potential difference between the contacts.
 11. An impressed current cathodic protection arrangement according to claim 10 in which the control unit comprises a transmitter for transmitting, to the central station, data indicating a measured potential difference between the contacts.
 12. An impressed current cathodic protection arrangement according to claim 11 in which the tool is arranged to transmit said data along the elongate metallic structure.
 13. An impressed current cathodic protection arrangement according to claim 10 in which the monitoring apparatus is arranged to determine the electrical potential of the structure relative to surroundings at at least one location which is spaced from said connection point in dependence on the potential difference measured between the spaced contacts of the tool as measured by the sensor of the tool.
 14. An impressed current cathodic protection arrangement according to claim 9 in which the tool is arranged for movement within the tubing.
 15. An impressed current cathodic protection arrangement according to claim 14 in which the monitoring apparatus is arranged to use the tool to take potential difference measurements one of continuously, and at intervals as it is moved along the tubing.
 16. An impressed current cathodic protection arrangement according to claim 1 in which the monitoring apparatus comprises a memory in which is stored a model based on parameters specific to the elongate structure and its surroundings which model predicts how the absolute value of a potential applied to the structure at the reference point will decay along the length of the structure away from the reference point.
 17. An impressed current cathodic protection arrangement according to claim 16 in which the monitoring apparatus is arranged to use the model based on parameters specific to the elongate structure and its surroundings in determining the electrical potential of the structure relative to surroundings at at least one location which is spaced from said connection point.
 18. An impressed current cathodic protection arrangement according to claim 2 in which the equipment for determining the electrical potential of the structure relative to surroundings at the reference point comprises a memory in which is stored a model based on parameters specific to the elongate structure and its surroundings which model predicts how the absolute value of a potential applied to the structure at the reference point will decay along the length of the structure away from the reference point.
 19. An impressed current cathodic protection arrangement according to claim 10 in which the monitoring apparatus is arranged to determine the electrical potential of the structure relative to surroundings at at least one location which is spaced from said connection point in dependence on: i) the electrical potential of the structure relative to surroundings at said reference point as determined by the equipment for determining the electrical potential of the structure relative to surroundings at the reference point; and in dependence on at least one of: ii) the potential difference measured between the spaced contacts of the tool as measured by the sensor of the tool; and iii) the model based on parameters specific to the elongate structure and its surroundings.
 20. An impressed current cathodic protection arrangement according to claim 1 in which the monitoring apparatus is arranged to indicate when the magnitude of the potential of the structure relative to the surroundings falls below a threshold level at at least one point along the length of the structure.
 21. An impressed current cathodic protection arrangement according to claim 1 in which the monitoring apparatus is arranged to indicate at what distance from the reference point the potential of the structure relative to the surroundings falls below a threshold level.
 22. An impressed current cathodic protection arrangement according to claim 1 in which the monitoring apparatus is arranged to indicate the potential of the structure relative to the surroundings as a function of distance away from the reference point.
 23. A well installation comprising an impressed current cathodic protection arrangement according to claim
 1. 24. A method of monitoring an impressed current cathodic protection arrangement, the arrangement comprising an elongate metallic structure to be protected, and cathodic protection apparatus which comprises a DC power supply and an anode, one terminal of the power supply being connected to the structure at a connection point and another terminal of the power supply being connected to the anode, the method comprising the step of: monitoring effectiveness of the cathodic protection provided by the cathodic protection apparatus by determining the electrical potential of the structure relative to surroundings at at least one location which is spaced from the connection point. 